This invention relates to a process for preparing a glass preform for production of an optical fiber. More particularly, this invention relates to an SiO.sub.2 glass preform doped with nitrogen.
Generally, a glass preform for optical transmission is required to have a predetermined distribution of refractive index in the radial direction of the glass rod, uniformity in the concentration and composition of ingredients, low content of OH radicals and impurities comprising transition metals such as iron and copper, and high light transmittance. As described in Japanese Patent Application (OPI) Nos. 6428/71, 5788/71, 10055/74 and 10056/74 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), such preform is conventionally produced by MCVD process, OVPO process or VAD process from silica-based glass doped with a metal oxide to provide high refractive index. However, the silica-based glass doped with metal oxide has high light transmittance but the dopant used is expensive. As taught in Japanese Patent Application (OPI) Nos. 76538/74 (equivalent to U.S. Pat. No. 4,082,420) and 87339/75 (equivalent to British Pat. No. 1,450,123 published Sept. 22, 1976), it is well known that oxide dopants can be replaced with fluorine or nitrogen to either decrease or increase the refractive index of glass, but this conventional method is not capable of producing stably silica-based glass containing a predetermined amount of nitrogen dopant. One of the articles that report the change in the refractive index of SiOxNy glass according to the amount of dopant nitrogen is A. K. Gaind and E. W. Hearn, "Physicochemical Properties of Chemical Vapor-Deposited Silicon Oxynitride from an SiH.sub.4 --CO.sub.2 --NH.sub.3 --H.sub.2 System" in J. Electrochem. Soc.: Solid-State Science and Technology, January 1978, pp. 139-145. One method for producing such SiOxNY glass is the chemical vapor deposition (CVD) process described in A. K. Gaind, G. K. Ackermann, V. J. Lucarini and R. L. Bratter, "Oxynitride Deposition Kinetics in an SiH.sub.4 --CO.sub.2 --NH.sub.3 --H.sub.2 System" in J. Electrochem. Soc.: Solid-State Science and Technology, April, 1977, pp. 599-606. However, the primary purpose of this method is to deposit a stable film of SiOxNy on a silicon wafer, and the method aims at providing an SiOxNy film of good characteristics rather than forming it quickly. For this reason, the rate at which the film is formed in a given period of time according to this method is low. That is, the method produces a non-defective film with low concentrations of materials being supplied into a heterogeneous reaction system at relatively low temperature. Contrary, in the production of a glass fiber for optical transmission, since the role of glass per se predominates over other components and also the glass is used in a large quantity, the rate of formation of a glass fiber must be at least a hundred times faster than that of the film of SiOxNy formed on the silicon wafer. The content of ##STR1## radicals in a glass fiber for optical transmission should be minimized because their presence is the cause of absorption loss, particularly transmission loss in the range of long wavelengths, due to their vibration. However, in the conventional film making technique that uses NH.sub.3 as one material, the formation of residual ##STR2## radicals is unavoidable because of the presence of hydrogen.
On the other hand, Japanese Patent Application (OPI) No. 134134/79 discloses that a porous glass can be doped with nitrogen by subjecting the porous glass to heat treatment in ammonia, but such process is not satisfactory since the amount of nitrogen doped is extremely low.